Development of the mammary gland during puberty, pregnancy, lactation and involution is controlled by growth modulators which function in a systemic, paracrine and autocrine fashion. We have shown that TGF-beta-1 and an endogenous milk protein, the whey acidic protein (WAP), are modulators of mammary development. Temporal deregulated expression of these proteins in mammary epithelial cells of transgenic mice resulted in abrogated mammary development. Furthermore, ectopic synthesis of the WAP under the control of the MMTV-LTR resulted in hyperplasias in the coagulation gland. No growth aberrations were observed in other expressing tissues, such as the salivary gland. This suggests that WAP exerts its growth modulatory effects in a cell-specific manner. The role of regulatory proteins in tissue formation and differentiation processes can be evaluated using 'gain of function' experiments in transgenic animals. However, the available systems do not permit a temporally controlled activation and inactivation of transgenes. We therefore established a binary system in which transcription of the transgene can be modulated by tetracycline. The reporter transgene (e.g. luciferase, beta-galactosidase or a growth modulatory) by itself is inactive because it is under the control of a basal promoter and seventetop sequences. Transcription of the reporter gene can be activated by a fusion protein containing the tetR and the activation domain VP16 from the Herpes simplex virus. This fusion protein is expressed under the control of a specific promoter/enhancer. Transcriptional activation can be controlled by tetracycline. The fusion protein binds to the tet/op only in the absence of tetracycline and stimulates transcription. In the presence of tetracycline the reporter gene is silent. such a system will be useful in regulating the temporal expression of growth factors and oncoproteins.